1. Field of Invention
This invention is in the field of laser scanners used for detection and measurement of three dimensional details on a target area.
2. Description of the Related Art
One challenge of scanning technology is that of translating the dimensions of an object into a computer compatible format. The idea is to acquire the object's image and digitize it for further digital manipulation. In this context, area scanners have been described to acquire the contours of an object by scanning the object and converting the image into a digital representation. For example, U.S. Pat. No. 5,969,822 to Fright et al. titled Arbitrary Geometry Laser Area Scanner describes an optical non-contact area scanning apparatus and method. The three dimensional coordinates of an illuminated profile or spot on the object are recorded and the area is reconstructed using spatial location and orientation information from a spatial location system associated with the object, illumination means and image recording means. The digitized output from Fright contains information about the image scanned by the apparatus.
Similarly, another example, U.S. Pat. No. 5,198,877 to Shulz titled Method and Apparatus for Three Dimensional Non-Contact Shape Sensing also describes sensing of numerous points on the area of an object utilizing two stages to sense a shape.
While both above systems and others in the prior art digitize an image into a computer file, at least two critical factors are only tangentially addressed, and need to be considered with respect to the present invention:
a) the time necessary to acquire the image inclusive of the time to process the acquired image information to detect small details in the image;
b) the degree of resolution to which the image has to be digitized to detect relatively small details within the image.
Typically, in the prior art, the object to be analyzed is presented to the image sensor, for example a Charge Coupled Device (CCD). The image of interest is contained in the output generated by the CCD, a large two dimensional array of individual pixel amplitudes. The output of the CCD is representative of the object presented to the camera and is processed to extract the image contained therein to create a usable computer file. The output from such a scanner is typically a large amount of data. For example, a digital image having 1024 by 1024 pixels, each pixel represented by 16 bits of amplitude to resolve small details, translates to approximately 16 million bits per image. The 16 million bits represent a substantial computational load, especially if thousands of images have to analyzed in minutes.
An approach towards reducing the time to process images of objects is to use an increased scan rate, that is, acquire more information per unit time. Such an approach is discussed in U.S. Pat. No. 6,031,225 to Stern et al, titled System and Method of Selective Scanning of an Object or Pattern Including Scan Correction. Here, an optical system quickly sweeps a light beam over the image to be acquired. A scan correction is applied to compensate for motion related errors.
However, even with the faster scan rate of the '225 patent, the rate of creating and analyzing images may be insufficient when a large number of images have to be captured and analyzed to detect small details in a short time. For example, where thousands of tiles each 3 in by 3 in have to be analyzed to detect relatively small details, such as area flaws in the order of 0.1 in by 0.1 in at a depth of 0.008 in, the prior art may not be satisfactory because of the delay involved to image the overall area of each tile and extract all features to the level where a small (0.1 by 0.1 by 0.008 in) detail is reliably detected. The prior art may require relatively large memories to store the output of high resolution analog to digital (A/D) converters. The high A/D resolution is needed to detect the small change corresponding to a small flaw in the object being examined, in the order of one part in 10000 (approximately 3 in/0.01 in). This means the computing load is quite large, and the analyzed results probably delayed, not available in real time. This is because a large amount of detailed (possibly 16 bit resolution) CCD data has to be stored, then processed to extract and detect the existence of small flaws on a large object. The large object is represented by a relatively vast digital output from the image(video) A/Ds, while the flaws are a small part ( 1/10000) thereof. Hence, the large computing load in detecting small flaws in a large digital image would delay output and the utility of a prior art system.